Journal of Composites for Construction

This paper presents the results of an experimental investigation into the behavior of slender steel columns strengthened using high-modulus $\left(313\mathrm{GPa}\right)$ , carbon fiber-reinforced polymer (CFRP) plates. Eighteen slender hollow structural section square column specimens, $44\times 44\times 3.2\mathrm{mm}$ , were concentrically loaded to failure. The effectiveness of CFRP was evaluated for different slenderness ratios $(kL∕r)$ , namely, 46, 70, and 93. The maximum increases in ultimate load ranged from 6 to 71% and axial stiffness ranged from 10 to 17%, respectively, depending on $kL∕r$ . As $kL∕r$ reduced, the effectiveness of CFRP plates also reduced, and failure mode changed from CFRP plate crushing after occurrence of overall buckling, to debonding prior to, or just at, buckling. A simplified analytical model is proposed to predict the ultimate axial load of FRP-strengthened slender steel columns, based on the ANSI/AISC 360-05 provisions, which were modified to account for the transformed section properties and a failure criteria of FRP derived from the experimental results. It was shown that for a given FRP reinforcement ratio, there is a critical $kL∕r$ at the low end, below which FRP may not enhance the strength of the column.

This paper presents the results of experimental program and analytical modeling for performance evaluation of a fiber-reinforced polymer (FRP) wrapping system to upgrade eccentrically loaded reinforced concrete (RC) columns. A total of 12 RC columns with end corbels were tested. The test specimen had an overall length of $\mathrm{1,200}\mathrm{mm}$ . Each end corbel had a cross section of $250\times 250\mathrm{mm}$ and a length of $350\mathrm{mm}$ . The specimen in the test region was $125\times 125\mathrm{mm}$ having a longitudinal steel ratio of 1.9%. Test parameters included confinement condition (no wrapping, full FRP wrapping, and partial FRP wrapping), and eccentricity-to-section height $(e∕h)$ ratio (0.3, 0.43, 0.57, and 0.86). Research findings indicated that the strength gain caused by FRP wrapping decreased as $e∕h$ was increased. Full FRP wrapping resulted in about 37% enhancement in compression strength at a nominal $e∕h$ of 0.3, whereas only 3% strength gain was recorded at a nominal $e∕h$ of 0.86. The compression strengths of the partially wrapped columns were on average 5% lower than those of the fully wrapped columns. A nonlinear, second-order analysis that accounts for the change in eccentricity caused by the lateral deformation was proposed to predict the columns strength. A comparison between analytical and experimental results of the present study in addition to data published in the literature demonstrated the accuracy and validity of the proposed analysis.

Rehabilitation and strengthening of concrete structures have become more common during the last $10–15\text{years}$ , partly due to a large stock of old structures and partly due to concrete deterioration. Also factors such as lack of understanding and the consequences of chloride attack affect the need for rehabilitation. In addition, more traffic and heavier loads lead to the need for upgrading. Existing externally bonded strengthening systems using fiber-reinforced polymers (FRP) and epoxy as bonding agents have been proven to be a good approach to repair and strengthen concrete structures. However, the use of epoxy bonding agents has some disadvantages in the form of incompatibilities with the base concrete. It is, therefore, of interest to substitute epoxy with systems that have better compatibility properties with the base concrete, for example, cementitious bonding agents. This paper presents a study on reinforced concrete beams strengthened in shear with the use of cementitious bonding agents and carbon fiber grids, denoted as mineral-based composites (MBC). In this study it is shown that the MBC system has a strengthening effect corresponding to that of strengthening systems using epoxy bonding agents and carbon fiber sheets. Different designs and material properties of the MBC system have been tested. An extensive monitoring setup has been carried out using traditional strain gauges and photometric strain measurements to obtain strains in steel reinforcement, in FRP, and strain fields on the strengthened surface. It has been shown that the use of MBC reduces strains in the steel stirrups and surface cracks even for low load steps as compared to a nonstrengthened concrete beam.

Experimental results of externally bonded strengthened beams, available to date in the literature, have shown the existence of some types of brittle failures that involve the premature laminate debonding before reaching the design load. Since the design procedure to obtain the laminate area to strengthen a reinforced concrete element should avoid these premature failures, there is a need to understand the mechanism of the debonding process in order to prevent it. This paper studies the evolution of the debonding process through the propagation of an interfacial crack by using nonlinear fracture mechanics assuming a bilinear constitutive law for the interface. The transference of stresses between the laminate and the concrete through the interface is first analyzed in a simple case of a pure shear specimen. Later on, this formulation is extended to a general case of a beam subjected to transverse loads. As shown in the analysis performed, the transferred force between both adherents should be limited to a maximum value to prevent laminate debonding. This theoretical maximum transferred force has been obtained for each case studied and will be the basis for the development of a future design proposal.

A prestressed carbon fiber-reinforced polymer (CFRP) strap retrofitting system has been found to significantly enhance the shear capacity of existing reinforced concrete beams. In previous studies, the CFRP straps were supported on metal pads placed on the top and bottom of a beam necessitating top surface access. The goal of the current work was to develop a system where the straps were installed from underneath a slab without compromising the strengthening efficiency. A series of T-beam experiments was conducted where the CFRP straps were inserted through holes that were drilled from below the flange, thereby avoiding the need for access to the top surface. The depth of penetration of the CFRP straps into the compression flange, the concrete strength, the CFRP strap spacing, the presence of holes in the compression flange, and the size of the loading pads were all found to affect the shear performance. Using the most successful installation technique, the resulting CFRP strengthened beam failed at a load that was approximately 50% higher than that of an unretrofitted control beam.

This study proposed a bond mechanism based deterioration model of bond interfacial fracture energy for fiber-reinforced polymer (FRP)-concrete joints in moist environments. The bond interface region relative humidity (IRRH) in a moist environment was correlated to the bond fracture energy in this deterioration model. The IRRH-dependent interface separation tractions were derived in the frame of a cohesive zone model. Such an IRRH-dependent interface separation-traction law was simulated by a series of nonlinear interface elements attached to the bond interface to calculate the macroscopic load-displacement curves for the modified double cantilever beam (MDCB) specimens. Through moisture diffusion analysis, IRRH was determined as a function of the moisture exposure time for given specimen dimension and environmental RH. Using IRRH as the bridge, the time-dependent load-displacement curves of the MDCB specimens were obtained. The good agreement with the experimental data indicated that the model worked well. The approach developed in this study can be used to simulate and predict the durability of the bond between the FRP and concrete in moist environments.